<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <script src="vue.js"></script>
</head>
<body>
  <div id="app">
    <div>{{a}}</div>
    <div>{{obj.b}}</div>
  </div>
  <script>
    var vm = new Vue({
      el: '#app',
      data: {
        a: '我是根级响应式属性a',
        obj: {}
      }
    })
    Vue.set(vm.obj, 'b', '我是Vue.set添加的响应式属性obj.b')
 </script>
</body>
</html>